My invention pertains to the design and construction of concrete slabs-on-grade which are used for tennis courts, driveways, highways, railway roadbeds and many other uses including interstate and airport pavements having heavy loads and frequent travel thereon. Such surfaces are hereinafter referred to as highway pavements. To overcome a multiplicity of quality and cost problems with the concrete slabs of the prior art, I have invented a simple and low-cost product and method for installing a total system which utilizes the tensile strength of the concrete rather than the strength of steel to maintain an acceptable surface for traffic.
Portland cement concrete (hereinafter referred to as concrete) has long been used for tennis courts, driveways, highways and airport runways because of its volumetric in-place economy and desirable physical properties. Hopefully, this concrete is a surface structure which supports the traffic whether the traffic is a tennis player, an industrial lift-truck, an automobile or a tractor-trailer truck. However, objectionable roughness from cracks and joints has impeded and slowed traffic and has required extra motive energy. Also roughness has accelerated the deterioration of vehicles. The results to date have been disheartening.
Unfortunately, prior to my invention, engineers have failed to design a tennis court as single concrete slab rather than a multiplicity of smaller slabs having joints or cracks between the slabs. In the prior art, a tennis court usually is a grouping of five to twenty slabs. Also highways typically have joints, by choice of the expert pavement design engineers, every 12, 20 or 45 feet and often cracks every 2 to 10 feet within several years of the time of installation.
Perhaps the most objectionable physical characteristic of concrete is a change of volume and dimension that is related to temperature and wetness. Concrete made using most cements, particularly those otherwise suitable for slabs, shortens with drying and a declining temperature as described in "Highway Engineering Handbook" and "American Concrete Institute" publications. Repeated shrinkage with falling temperature occurs after rising temperatures cause concrete expansion.
This objectionable shrinkage of concrete causes shrinkage cracks when a concrete pavement as large as a tennis court is "connected" to its supporting structure, as in prior art pavements, and is restrained during shrinkage to such an extent that the tensile force exceeds the tensile strength in the concrete. The result is a cracked slab. Of course, any traffic load on the slab is an added consideration because the restraint tension and the bending tension on the lower surface are additive.
The placing of reinforcing steel bars in the concrete is of no merit in preventing shrinkage cracks because the steel cannot shrink like the concrete during concrete drying and, as a result, the steel further restrains the concrete during drying shrinkage. Such steel actually increases shrinkage cracks.
The prior art in respect to concrete slab-on-grade restraint is exemplified by Subcommittee IV, Committtee 325 report published in "Journal of the American Concrete Institute" V28, No. 4, October, 1956. A coefficient of friction of 1.5 is said to be typical for the restraint between slabs and their base. Attempts to obtain lower restraint of slabs have included use of a sand layer one inch thick below the slab with the coefficient remaining near 1.0. Later Pennsylvania Department of Transportation in 1973-74 near Hogestown installed concrete slabs over two layers of polyethylene film and thereby expected to lower the coefficient of friction to a 0.65 to 1.0 range although the data leave the actual friction coefficient obscure.
Foulger U.S. Pat. No. 3,000,276 suggests the use of at least two sheets of polymeric film (high density polyethylene) between the base and the concrete slab. The polyethylene films may have a slip agent in the polymer so as to give an interfacial coefficient of friction of the order of 0.24 to 0.13. The Foulger patent suggests that the polyethylene sheet assembly of two sheets may consist of a length of flattened tubing.
The prior art appears silent on a method for reducing slab restraint involving reduced machanical interlocking of the slab on its support structure. A plan was underway in Mississippi in 1976 to use an aggregate-bituminous mix below a slab. This mix should be an aid toward less slab restraint but still in the 0.6 to 1.0 range of friction coefficient is expected.
By making slab dimensions small, the typical tennis court and highway builder has avoided much of the shrinkage crack problem but at the same time has incurred a slab with many joints plus possible cracks. Joints and cracks normally require some means for sealing out rainwater and for vertical alignment. Hence serious new problems result from smaller slabs. Because water enters the support structure at joints and cracks and destroys the firmness of that support, the pavement in the zone near joints and cracks cannot support traffic loads the same as at the middle of a slab. Even major upgrading of the underlying support structure causes a serious cost problem and does not solve the problems caused by cracks and joints.
A recent development of merit is prestressed concrete slabs wherein relatively large prestress forces are used to close cracks tightly and to make the cracks non-functionable thereby reducing the water sealing and vertical alignment problems except at the joints every 400, 500 or 600 feet. To aid in obtaining the desired prestress force such slabs also have been placed on two layers of polyethylene.
In all cases where two or more sheets of polymeric film are used the desired results might be obtained as long as the annual frost line is less than the thickness of the concrete slab. However, in areas where the annual frost line is greater than than thickness of the slab the freezing of water that accumulates as liquid or vapor between the layers of polymeric film tacks the two sheets of film together so that the desired result of reduced friction and low restraint between the base and the concrete slab is lost. This occurs even with the flattened tubing suggested by the Foulger patent as the tubing edges are torn when the concrete slab expands and contracts while resting on one side of the tube.
All of these attempts have not solved the problem of obtaining slabs-on-grade that have satisfactory in-service smoothness at desirable cost. Perhaps the best source of information on prior art is "Highway Engineering Handbook", K. B. Woods, Editor-in-Chief, First Edition 1960, McGraw-Hill Book Company, New York, N.Y.